Typha for paludiculture—Suitable water table and nutrient conditions for potential biomass utilization explored in mesocosm gradient experiments

Abstract Drainage has turned 650,000 km2 of peatlands worldwide into greenhouse gas sources. To counteract climate change, large‐scale rewetting is necessary while agricultural use of rewetted areas, termed paludiculture, is still possible. However, more information is required on the performance of suitable species, such as cattail, in the range of environmental conditions after rewetting. We investigated productivity and biomass quality (morphological traits and tissue chemical composition) of Typha angustifolia and Typha latifolia along gradients of water table depth (−45 to +40 cm) and nutrient addition (3.6–400 kg N ha−1 a−1) in a six‐month mesocosm experiment with an emphasis on their high‐value utilization, e.g., as building material, paper, or biodegradable packaging. Over a wide range of investigated conditions, T. latifolia was more productive than T. angustifolia. Productivity was remarkably tolerant of low nutrient addition, suggesting that long‐term productive paludiculture is possible. Low water tables were beneficial for T. latifolia productivity and high water tables for T. angustifolia biomass quality. Rewetting will likely create a mosaic of different water table depths. Our findings that the yield of T. angustifolia and tissue chemical composition of T. latifolia were largely unaffected by water table depth are therefore promising. Depending on intended utilization, optimal cultivation conditions and preferable species differ. Considering yield or diameter, e.g., for building materials, T. latifolia is generally preferable over T. angustifolia. A low N, P, K content, high Si content and high C/N‐ratio can be beneficial for processing into disposable tableware, charcoal, or building material. For these utilizations, T. angustifolia is preferable at high water tables, and both species should be cultivated at a low nutrient supply. When cellulose and lignin contents are relevant, e.g., for paper and biodegradable packaging, T. angustifolia is preferable at high water tables and both species should be cultivated at nutrient additions of about 20 kg N ha−1 a−1.


| INTRODUC TI ON
Peatlands are one of the most effective terrestrial ecosystems to sequester carbon (C) as organic matter (Knox et al., 2015;Rocha & Goulden, 2009). Globally, 650,000 km 2 of peatlands have been drained, mainly for agriculture, and thereby turned into carbon sources (Joosten & Clarke, 2002). Therefore, peatland restoration is a very effective measure for greenhouse gas (GHG) emission mitigation (Leifeld & Menichetti, 2018). To substantially reduce net GHG emissions from the peatland biome, all drained peatlands need to be rewetted and large-scale rewetting should consequently start now (Günther et al., 2020;Leifeld et al., 2019;Tanneberger et al., 2021).
Conventional drainage-based agriculture is no longer possible on rewetted peatlands. To enable a sustainable use of the vast areas, alternative practices such as their wet use, termed paludiculture (Wichtmann et al., 2016a), have to be explored. However, the response of target crops to the variety of environmental conditions after rewetting, especially to higher nutrient loads and stronger water table fluctuations than under natural peatland conditions , are not yet investigated comprehensively.
Suitable target crop species for paludiculture are adapted to wet conditions and their biomass utilization is economically viable (Abel et al., 2013). Typha spp. fulfill these requirements: they can grow under waterlogged and even flooded conditions and produce more dry matter (>20 t dry matter ha −1 a −1 reported for North America and South Germany  or about 10-15 t dm ha −1 a −1 on rewetted semi-natural sites in our study region in Midwest Europe (Geurts et al., 2020;Zerbe et al., 2013)) than conventional intensive grassland use on the same sites while drained (11-14 t dm ha −1 a −1 ; Geurts & Fritz, 2018). N-availability for economically viable Typha paludiculture is suggested to be 100 kg N ha −1 a −1 (Geurts & Fritz, 2018), which is lower than the input from fertilization and net N mineralization under grassland use in drained conditions (up to 340 kg N ha −1 a −1 from fertilization and up to 600 kg N ha −1 a −1 from net N mineralization; Schrautzer, 2004;Tiemeyer et al., 2016). Thereby paludiculture can reduce the risk of eutrophication of downstream water bodies.
Typha paludiculture furthermore offers a large range of biomass utilization options.
Typha biomass is well suited as a building material mainly for construction and insulation as about 85% of its tissue consist of aerenchyma, giving the material good insulation properties, a low density, and simultaneously high break resistance and bearing capacity (Georgiev et al., 2014;Krus et al., 2014). Climate benefits associated with the use as building material are emission reduction during cultivation, substitution of fossil raw materials, and long-term carbon storage in the products (Lahtinen et al., 2022;Pittau et al., 2018).
For the use as building material plants must, however, fulfill certain requirements including, but not limited to their morphological properties, such as height and diameter. While a low water content is desirable (Wichtmann et al., 2016b), Typha plants keep a high water content of up to 32%-60% at winter harvest (Maddison, Soosaar, et al., 2009) and biomass has to be predried for storage and most applications (Geurts et al., 2020;Geurts & Fritz, 2018). Paper and biodegradable packaging are other promising high-value utilizations (Alebiosu et al., 2019;Reich, 2019). For these materials, high cellulose and low lignin contents are required and plant fiber dimensions may be of importance (Ververis et al., 2004). However, the biomass quality requirements for different utilizations remain unknown and vague in many cases, the reasons being that Typha biomass is rarely used as raw material or utilizations are still in a test phase.
In field cultivation, many factors can influence the productivity and biomass quality of crops, including species, genotype, water table, nutrient supply, and harvest date (Geurts et al., 2020;Kasak et al., 2020;McNaughton, 1974;Pijlman et al., 2019). Especially water tables and nutrient supply can be difficult to control in large-scale paludiculture, implying that farmers will need to optimize their crop selection and its usage under the given conditions. Yet, research directly comparing Typha species and the quantity and quality of their biomass production under various environmental conditions is rare. Previous studies with Typha often include only one species (Li et al., 2004;Ren et al., 2019;Wetzel & van der Valk, 1998) or focus on either nutrient or water supply (Deegan et al., 2007;Hong et al., 2020;Macek & Rejmánková, 2007).
Typha angustifolia (T. angustifolia) and Typha latifolia (T. latifolia) are the most relevant Typha species for paludiculture in Germany. They occur over a wide range of nutrient availability (growing at total N additions of 0-1330 kg N ha −1 a −1 under controlled conditions (Geurts & Fritz, 2018;Ren et al., 2019;Wild et al., 2001)). T. angustifolia is often found to be more productive than T. latifolia under field and experimental conditions Grace & Wetzel, 1981).
Typha species are highly productive under waterlogged conditions and show decreased productivity when facing prolonged (water table 30 cm below soil surface) or periodic (soil water potential down to −0.8 Mpa) soil moisture deficits (Geurts & Fritz, 2018;Li et al., 2004). T. latifolia outcompetes T. angustifolia in shallow water (<50 cm) but T. angustifolia is better adapted to growing in deeper water depths (Grace & Wetzel, 1981, 1982. While both

T A X O N O M Y C L A S S I F I C A T I O N
Agroecology; Applied ecology; Botany; Ecological economics; Environmental sustainability; Functional ecology species develop aerenchyma as an adaption to flooding (Constable et al., 1992;Ni et al., 2014), flooding may lead to light limitation for photosynthesis, which can be counteracted by increased height growth (Grace, 1989;Grace & Wetzel, 1982;Heinz, 2012). The effect of water table on biomass production is not clear: Some studies report no effect of flooding on total biomass (Byun et al., 2017;Hong & Kim, 2016) while others show the highest productivity of young Typha plants under water tables at soil surface but reduced biomass production at lower or higher water tables (Heinz, 2012).
To optimize Typha-based paludiculture on rewetted peatlands as a profitable alternative to drainage-based agriculture, we need to increase our understanding on how Typha species perform under a wide range of environmental conditions. Therefore, we conducted a mesocosm study in which we directly compared the performance of T. angustifolia and T. latifolia along a water table gradient and a nutrient addition gradient in two parallel experiments. We quantified belowground biomass production, which is the basis for peat production, C sequestration, and growth in the following season (McNaughton, 1974;Rocha & Goulden, 2009;Schwieger et al., 2020;Succow & Joosten, 2001), physiological activity (photosynthesis), and the quantity and quality (morphological traits and tissue composition) of aboveground biomass.
We hypothesized for both species that (1a) biomass productivity reaches an optimum at water tables close to soil surface and photosynthetic rate decreases with increasing water tables and (1b) biomass quality for high-value utilization is optimal at high water tables as this should result in higher physical and chemical resistance to physical disturbance (water/wave movement) and rotting (low N, P, K content, high Si content). We furthermore expected (2a) biomass productivity and photosynthetic rate to increase with increasing nutrient addition and (2b) biomass quality to generally decrease with increasing nutrient addition (high N, P, K content) with notable exceptions such as stem diameter and potentially also Si content being positively influenced by increasing nutrient addition. With regards to each of the two species, we hypothesized that (3a) T. latifolia reaches its optimum performance at lower water tables than T. angustifolia as the latter was expected to have its optimum performance at higher water tables, that (3b) the two species reach optimal performance at a similar nutrient addition and that (3c) T. angustifolia generally outperforms T. latifolia.
Finally, we discuss the implications of our results for the use of Typha spp. in paludiculture of rewetted temperate fen ecosystems.

| MATERIAL AND ME THODS
We grew two species of cattail, Typha angustifolia and Typha latifolia, in a mesocosm experiment and assessed their performance along gradients of (A) nutrient addition and (B) water table height.

| Experimental setup
We collected seeds of both Typha species in February 2019 in a naturally established stand near Lake Kummerow in Mecklenburg-Western Pomerania. In April 2019, seeds were sown on trays with an equal mixture of sand, loamy soil, and commercial substrate (Naturtalent by toom® Bio-Universalerde, toom Baumarkt GmbH, Köln, Germany). Trays were kept in a greenhouse at 22°C and soil was kept moist at all times using tap water. Trays were rotated regularly to ensure equal conditions for all plants. In May 2019, plants were potted into multi-pot plates using bog peat as substrate (Sphagnum peat, limed, not fertilized, pH 5.6-6.4; Torfwerk Moorkultur Ramsloh, Germany; element content in % dry matter (DM): 0.95% DM total N (Kjeldahl method), 0.026% DM total P (Aqua regia digestion, ICP-OES), 0.06% DM K (Aqua regia digestion, ICP-OES), 0.15% DM Mg (Aqua regia digestion, ICP-OES), 0.13% Fe (Aqua regia digestion, ICP-MS)) and transferred outside. For the water table treatments, plants were grown in a gradient of 15 water tables ranging from a water table 45 cm below soil surface to 40 cm above soil surface ( Table 1). Tubes were closed at the bottom with two layers of water permeable root fleece (polypropylene, 150 g m −2 ). Tubes were placed on wooden platforms inside 1000 L water containers (1 × 1 × 1 m) to achieve the desired water tables. In each container, two different water levels were simulated by constructing wooden platforms of different heights ( Figure 1).
While all water tables below soil surface were kept constant from the beginning of the experiment, water tables above soil surface were raised according to plant growth until the intended water table was reached, which was the case for all levels at the beginning of August. To provide the plants with the macronutrients (N, P, and K), they were fertilized with a nutrient solution amounting to 37.9 kg N ha −1 a −1 equivalent to the middle level (Level 8) of the nutrient addition treatment, which was prepared in the way described below. The water table treatments were fertilized bi-weekly starting on 11 th June 2019. At the first fertilization, a third of the yearly nutrient dose was given, and the remaining amount of nutrients was split over the remaining six dates. 0.5 L of nutrient solution was poured directly into each tube (water level at or below soil surface) or in the water above each tube (water levels above soil surface).
For the nutrient treatments, plants were grown in a nutrient addition gradient, ranging from addition of 3.6-400 kg N ha −1 a −1 in a nutrient solution, not taking into account additional nutrient sources such as the substrate, water, or atmospheric deposition. The gradient consisted of 15 levels with increasing nitrogen (N) addition by a factor of 1.4 ( Table 2). In addition to nitrogen, phosphorus (P) and potassium (K) were supplied. We chose an N/P-ratio of 10 as previous studies by Tylová et al. (2013) and Romero et al. (1999) found this ratio to be suitable for the growth of emergent wetland plants.
We chose an N/K-ratio of 1.452, according to measurements of pore water in fens across central to eastern Europe (Tanneberger, 2019).
Tubes were sealed at the bottom, and the water table was kept at soil surface constantly. Each treatment had a separate water reservoir to avoid mixing of nutrient concentrations among treatments ( Figure 1). For the nutrient addition, the target amount of nitrogen was scaled down to the surface area of a tube (314.16 cm 2 ) and then split over 7 dates at intervals of 2 weeks throughout the growing season. At the first fertilization (11 th June 2019), a third of the yearly nutrient dose was given, and the remaining amount of nutrients was split over the remaining six dates. The nutrient solution was prepared by dissolving NH 4 NO 3 , (NH 4 ) 2 HPO 4 , and K 2 CO 3 in demineralised water ( Table 2). For a nutrient addition treatment, the water table in the tubes was lowered to allow the nutrient solution to penetrate the soil. Then, 0.5 L of nutrient solution was poured into each tube. After the first nutrient addition, some Typha plants in the higher nutrient addition treatments died. All dead plants were re-

| Data collection
We measured the number of leaves on the highest shoot on each plant in mid-September at the end of the growing season. Plants Water content of aboveground biomass was calculated from its fresh and dry weight according to Formula 2.
C/N-ratio was calculated as a mass-based C/N-ratio.
Belowground biomass was washed, separated into roots and rhizomes (per tube), and dried at 60°C until constant dry weight. We recorded the dry weight of roots and rhizomes at 0.01 g precision. (1) In the nutrient addition gradient ((c) side view, (d) top view), tubes with T. angustifolia and T. latifolia of each level were connected to their own water reservoir (gray buckets). Fifteen levels formed a gradient of nutrient addition ranging from the addition of 3.6-400 kg N ha −1 a −1 . In reality, gradient levels were arranged randomly. When it was not possible to measure on the second fully expanded leaf from the top and another leaf was chosen, photosynthetic rate From these values, the mean photosynthetic rate per level and species was calculated.

| Statistical analysis
This study was designed as a gradient experiment (Kreyling et al., 2018), aiming to observe patterns of response variables over gradients of the environmental drivers nutrient availability and water table. To unravel these nonlinear response patterns, a graphical analysis was performed in R (Version 3.6.0; R Core Team, 2019) using the package ggplot2 (Version 3.3.0; Wickham, 2016). Each response parameter was plotted over the environmental gradient and smoothed conditional means were calculated by Local Polynomial Regression Fitting using the "loess" function implemented in R. Span was adjusted to produce smooth curves without multiple extrema. This was done based on the assumption that several minima and maxima along the gradients would not be ecologically plausible in such an autecological setting without competition. Response values reported in the results were obtained from the Local Polynomial Regression Fitting.
Confidence intervals (CIs) displayed around the conditional means were used to assess significant effects at a level of α = 0.05. The effect of an environmental driver on the response parameter was considered significant if a straight horizontal line could not be fitted inside the 95% CI (Gelman & Hill, 2007). The species were considered significantly different if their 83% CIs did not overlap as the comparison of two 83% CIs approximate an alpha level of 5% (Austin & Hux, 2002;Knol et al., 2011;Payton et al., 2003).

| Practitioner survey
In April and May 2021, an international survey was conducted to collect information from practitioners about biomass requirements for various utilization options. Prior to the survey, companies already using paludiculture biomass or other plant biomass as raw material for their products were identified. A questionnaire was designed with questions regarding currently used raw materials, quality requirements for raw materials, manufactured products, willingness to substitute other plant-based raw materials with paludiculture biomass (e.g., Typha spp., Phragmites australis, wet meadow grasses), limiting factors to use paludiculture biomass, required volume and processing state, and price expectations. In this publication, we focused on statements on quality requirements and refrained from evaluating the other topics.

| Water table gradient
Total biomass production of T. angustifolia was not affected by water table, whereas T. latifolia produced more biomass at water tables below and close to soil surface, and its biomass production shoot height 146 cm at +40 cm) and more leaves per shoot (max. 8 leaves at +40 cm) under flooded conditions. T. angustifolia grew significantly taller than T. latifolia under flooding (approx. +20 to +40 cm), but T. latifolia produced more shoots and more leaves per shoot than T. angustifolia over a large part of the water table gradient (span = 2.5), and (g) maximum clum diameter [cm] (span = 1.5) at harvest of Typha angustifolia (blue) and Typha latifolia (red) along the water table gradient. Negative water tables are below soil surface and positive water tables above soil surface. Dots are original data points, and lines show the smoothed local polynomial regression fitting (loess). 95% confidence intervals are displayed in gray, and 83% confidence intervals are displayed in color. Shaded triangles next to plots indicate preferred (=dark) biomass properties for high-value utilization.
(approx. −40 to −5 cm and +5 to +35 cm; approx. −40 to +25 cm, respectively). The maximum culm diameter of both species did not change over the water table gradient (Figure 2g). The diameter of T. latifolia (1.3-1.7 cm) was significantly and up to 1.7 times higher than the diameter of T. angustifolia (0.9-1.1 cm).
Photosynthetic rate of T. latifolia decreased significantly from dry to flooded conditions while photosynthetic rate of T. angustifolia did not change over the water table gradient (Figure 2d). T. latifolia Water content of aboveground biomass increased in both species significantly towards more flooded conditions (Figure 3h). T. latifolia showed an increase from 61% to 88% and T. angustifolia from 67% to 86%. At water tables below ground (approx. −25 to −42 cm), T. angustifolia had a higher water content than T. latifolia; however, T. latifolia had a higher water content than T. angustifolia at water tables above ground (approx. +15 to +25 cm).

| Nutrient gradient
Total biomass of both Typha species did not change significantly at low to intermediate nutrient addition but decreased significantly at high nutrient addition (Figure 4a). The same pattern was observed for above-and belowground biomass (Figure 4b,c), and root and rhizome biomass ( Figure A4). Only rhizome biomass of T. latifolia showed a significant increase from low to intermediate nutrient addition, with an optimum at 29 kg N ha −1 a −1 . T. angustifolia reached a maximum total biomass productivity of 77 g (aboveground: 25 g; belowground: 52 g) at a nutrient addition of 13 kg N ha −1 a −1 while T. latifolia reached a maximum total biomass productivity of 118 g (aboveground: 44 g; belowground: 74 g) at a higher nutrient addition of 27 kg N ha −1 a −1 .
T. latifolia was significantly more, and up to 3.3 times as productive as T. angustifolia in a range of approx. 12-180 kg N ha −1 a −1 , but the productivity of the species did not differ significantly at low and very high nutrient additions. The differences in productivity between the species were more pronounced in aboveground biomass compared to belowground biomass.
Average plant height and shoot number at harvest (Figure 4e,f) and leaf number at the end of the growing season ( Figure A5 Dots are original data points, and lines show the smoothed local polynomial regression fitting (loess). 95% confidence intervals are displayed in gray, and 83% confidence intervals are displayed in color. Shaded triangles next to plots indicate preferred (= dark) biomass properties for high-value utilization. Missing values are due to the fact that sufficient biomass was lacking for the analyses because the plants were growing poorly or had died. and C/N-ratio decreased significantly with increasing nutrient addition ( Figures A6 and 5g). Si concentrations showed no significant response (Figure 5d).
Nutrient addition had no influence on cellulose content in either species and species did not differ in cellulose content (Figure 5e).
Lignin content decreased significantly with increasing nutrient addition in both species (Figure 5f). In T. angustifolia, lignin content already decreased from low nutrient addition onwards while in T. latifolia, it was stable up to approx 20 kg N ha −1 a −1 , then decreased. At low nutrient addition (approx 3.6-8 kg N ha −1 a −1 ), lignin content was significantly higher in T. angustifolia than in T. latifolia.
The water content of aboveground biomass increased significantly for T. angustifolia over the entire nutrient gradient from 65% to 82% (Figure 5h). T. latifolia showed an increase from 72% at 11 kg N ha −1 a −1 to 84% at high nutrient addition.

| Practitioner survey
In total, 39 companies were contacted of which five responded with a fully completed questionnaire. All companies made a statement of preferred water content and almost all agreed that little or no other vegetation (or other impurities) should be included in the required biomass ( Table 3). Only two companies required certain morphological parameters and two companies a certain chemical composition of the biomass for their products. The other companies did not respond or gave the following reasons for not having filled in the questionnaire: No time for a questionnaire, trade or sale of products only (no production), and no statements about biomass requirements can be made.

| DISCUSS ION
The concept of paludiculture has been proposed as a sustainable land-use option for peatlands for a long time already (Wichtmann & Joosten, 2007), yet large-scale practical implementation is still at the beginning. The reasons for this are manifold, including the political framework, economic uncertainties but also lack of knowledge on the selection of suitable species or varieties for given environmental conditions. Our aim in this study was to investigate under which environmental conditions the Typha species T. latifolia and T. angustifolia perform best regarding their aboveground and belowground productivity and aboveground biomass quality for high-value utilization to provide references for practitioners to select the appropriate species for specific field conditions and intended use.

| Performance of Typha spp. in the water table gradient
The optimum of biomass production was, contrary to hypothesis 1a, not at water tables close to soil surface. Productivity of T. angustifolia TA B L E 3 Results of the practitioner survey. Five companies responded with a fully completed questionnaire regarding currently used plant biomass, products, and biomass quality requirements  Geurts & Fritz, 2018;Li et al., 2004) due to its shallow root system being restricted mainly to the upper 15-30 cm of the soil (Geurts & Fritz, 2018). The fact that plants still received rainfall and bi-weekly fertilizer solution might have reduced the water stress in our experiment by elevating soil moisture in the mesocosms. Water tables, however, fluctuated only within 1-2 cm even after the strongest rainfall events. Macronutrient concentrations in plant biomass, as a measure of plant available nutrients (Ławniczak, 2011), did not correlate with productivity in either species, indicating that higher nutrient concentrations in the soil water of dry compared to flooded mesocosms (Table A1) did not drive the observed pattern in productivity.
In our study, water table did not affect the performance of T. angustifolia, but that of T. latifolia, whose productivity and photosynthetic rate decreased significantly with increasing water table.
Flooding (approx. from +20 cm), however, led to an increase in T. angustifolia plant height, which has been shown for several Typha species (Grace, 1989;Grace & Wetzel, 1982;Heinz, 2012). Although we did not see a clear optimum in performance of T. angustifolia at high water tables, the species seems to cope better with flooding than T. latifolia, a fact that is supported by previous studies (Grace & Wetzel, 1981). Additionally, it was found that T. latifolia could not aerate its substrate significantly (Grace, 1988). In competition, T. latifolia can outcompete T. angustifolia in shallow water because of a greater leaf surface area, but T. angustifolia is well-adapted to growing in deeper water depths due to its narrow leaves and large rhizome storage (Grace & Wetzel, 1981, 1982. Our hypothesis 3a can only be confirmed in so far as T. latifolia showed optimum performance at low water tables and T. angustifolia could cope well with high water tables, but we could not confirm different optima of performance along the water table gradient for the two species since T. angustifolia did not show an optimum in performance.

| Performance of Typha spp. in the nutrient addition gradient
Biomass and morphology, as well as photosynthetic rate of both Typha species were, contrary to hypothesis 2a, largely unaffected by low to intermediate nutrient addition but declined at high nutrient addition. Likewise, Ren et al. (2019) and Geurts et al. (2020) reported that T. latifolia can perform well at low nutrient addition. Our results suggest that this tolerance to low nutrient levels does not only apply to T. latifolia but also to T. angustifolia. It should be noted that the nutrient addition values in our study only refer to the nutrient addition with the fertilizer solution, excluding additional nutrients from the substrate, irrigation water, or atmospheric deposition. Hence, actual nutrient availability was likely higher than the indicated nutrient addition. Atmospheric deposition is estimated to add approximately 10 kg N ha −1 a −1 in our study area (Erisman et al., 2015), roughly doubling the lowest nutrient addition level over the course of the experiment. To keep the additional nutrient supply by the substrate as low as possible, we used nutrient-poor bog peat instead of fen peat.
Performance of T. latifolia and T. angustifolia had different optima along the nutrient addition gradient, contradicting hypothesis 3b.
Optima of productivity occurred at lower nutrient concentrations in T. angustifolia than in T. latifolia. This could be related to the fact that the performance of T. angustifolia remains stable at low nutrient addition while performance of T. latifolia shows a nonsignificant trend to decrease. It could also be that T. latifolia has a higher tolerance to stress at high nutrient addition than T. angustifolia.
In contrast to the decrease in plant performance observed in both Typha species at high nutrient addition in our study, biomass production and photosynthetic rate are often found to be enhanced by an increased nutrient availability even beyond the levels applied in our study (Deegan et al., 2012;Geurts & Fritz, 2018;Grace, 1988;Jordan et al., 1990;Macek & Rejmánková, 2007;Miao et al., 2000;Ren et al., 2019;Santos et al., 2015;Svengsouk & Mitsch, 2001;Wetzel & van der Valk, 1998). Previous studies have shown that high nutrient concentrations (>100 g L −1 ammonia) can be harmful to Typha and induce morphological and physiological changes in the plants (Chikov et al., 2012;Clarke & Baldwin, 2002;Hong et al., 2020;Jinadasa et al., 2008;Li et al., 2011). In a study with maize, Magalhães et al. (1995) found that plant growth was lower with NH + 4 -than with NO − 3 fertilization under low oxygen supply. In the NH + 4 fertilized plants they observed higher levels of free NH 3 , which was negatively correlated with growth. It is known that NH 3 is toxic to plants (Vines & Wedding, 1960). The equilibrium between NH + 4 and NH 3 depends on pH and temperature. At a temperature of 30°C and pH of 9.5, as measured in our fertilizer solution, it is heavily shifted towards NH 3 (Kadlec & Wallace, 2009). Considering this fact, and the anoxic conditions in the mesocosms, the unpleasant smell of the fertilizer solution and unspecific symptoms of plants, yellow leaves, becoming soft and dying quickly lead us to suspect that the fertilizer solution had an elevated NH 3 content, which led to NH 3 poisoning of the plants in the levels with high nutrient addition. It seems unlikely that such elevated NH 3 concentrations occur under field conditions and should be kept in mind when interpreting the performance of Typha in this part of the gradient.

| T. latifolia outperforms T. angustifolia
In contrast to hypothesis 3c, T. latifolia outperformed T. angustifolia regarding productivity, growth, and photosynthetic rate throughout the range of nutrient availability and water table depths of our experiments. This is in contrast to North American studies, which report T. angustifolia to be more productive than T. latifolia under experimental (Grace & Wetzel, 1981) and field conditions .

Under field conditions, T. angustifolia can develop denser stands
with more shoots per area , which could account for the higher yield compared with T. latifolia in the studies mentioned. Due to limited space in our mesocosms, rhizomes could not spread naturally but grew curled up inside the plastic tubes, which may have affected the observed shoot density. It is also possible that stand age plays a role (Geurts et al., 2020;Tanaka et al., 2004) or that Typha genotypes differ inherently, as is known from Phragmites australis (Achenbach et al., 2012).
Despite young plant age, we observed yields of 9.1-20.4 t dm ha −1 a −1 for T. latifolia and 2.3-11.9 t dm ha −1 a −1 for T. angustifolia in the water table gradient and of 0.4-16.7 t dm ha −1 a −1 for T. latifolia and 0.2-10.9 t dm ha −1 a −1 for T. angustifolia in the nutrient addition gradient.
These yields are in the same order of magnitude as in field studies and maximum yields are relatively high compared with studies in Central Europe (Geurts et al., 2020;Maddison, Soosaar, et al., 2009;Zerbe et al., 2013) possibly due to favorable conditions in the mesocosms.

| Preferable conditions vary depending on intended biomass use
The biomass requirements of Typha species for utilization options have to be identified to enable farmers to produce biomass of suitable quality. However, only few requirements are established. We conducted a practitioner survey to gain more insights into quality requirements to better link research and practice. While the participants could give clear information on preferred water content and morphology, notably there were less clear requirements for element content that were not quantified.
In the following, we attempt to evaluate the results from our experiments for potential utilizations based on the results of the practitioner survey and findings from the literature.
Some building materials and their production processes require, among other criteria, specific morphological parameters such as large plants and a certain shoot diameter (>4 cm diameter for the TYPHABOARD, (Georgiev et al., 2019;Krus et al., 2014;Theuerkorn, 2022), >2 × 2.5 cm diameter of elliptical shoots for building material (survey, Table 3)). In our experiments, both species did not reach these targets, probably due to their age (first growing season) and spatial limitation in the tubes. Under field conditions, T. latifolia can reach a diameter of 10 cm (2 nd growing season, unpublished own data) and T. angustifolia can reach a diameter of 5 cm (Asaeda et al., 2005). In our study, T. latifolia had a greater diameter than T. angustifolia under most conditions and would therefore be the preferred species for the above-mentioned building materials, based on diameter. Water level and nutrient addition did not influence culm diameter in T. latifolia, while the maximum culm diameter of T. angustifolia decreased significantly with increasing nutrient addition. This is in contrast to other studies such as Ren et al. (2019), where, among other traits, maximum shoot diameter increased with increasing nutrient availability (up to 300 kg N ha −1 a −1 ) in T. latifolia. Plants not fulfilling diameter requirements for building material and insulation boards can still be used for products that do not require specific morphological properties, e.g., blow-in insulation.
Chemical composition of the biomass can play a role in different utilization pathways: For processing into disposable tableware, a low mineral content of raw biomass can be beneficial (Table 3).
For processing into charcoal, a high C content is desirable ( Table 3).
For bio-based building material, the content of macronutrients N, P, and K in the biomass seems to have no relevant influence, as long as it is used in dry surroundings, e.g., as indoor insulation material.
In a humid environment, however, e.g., due to leakage and inflow of humid air , a low nutrient content in the biomass can possibly be desirable for lower degradability (e.g., if the material is not treated with chemicals), as is known for common reed as thatching material (Greef & Horlings, 2016). Higher biomass C/N-rates can further be beneficial for durability, as they are negatively correlated with decomposition (Chimney & Pietro, 2006).
Additionally, a high Si content is favorable as it leads to building material that is more durable, resistant to decay, less flammable, and insulates better against heat and cold (Vasanthi et al., 2012).
In the water table gradient, the two species showed different patterns of biomass nutrient content. While nutrient content of T. latifolia stayed in a narrow range over the gradient, nutrient content of T. angustifolia decreased significantly at high water tables. Biomass nutrient content increased for both species with increasing nutrient addition. In both species, Si content did not change over the nutrient addition gradient but increased 2-to 3-fold at high water tables. We assume that high water tables lead to increased uptake and storage of Si in the plant tissue of Typha to increase the plants' stability.
Assuming that a low content of N, P, K, a high content of C, Si, and high a C/N-ratio are beneficial, we conclude that biomass quality in terms of element content decreases with increasing nutrient availability and, in particular for T. angustifolia, increases with increasing water table. The trade-off between increasing productivity but decreasing biomass quality with increasing nutrient addition emphasizes the importance of adapting the utilization depending on site nutrient availability.
A low water content in aboveground biomass is desirable for storage and processing into various products (Table 3). To reduce drying time and effort, a low initial water content is preferable.
At water tables between 0 and +20 cm, the water content of aboveground biomass harvested in our experiment matches findings from field harvests in late autumn .
Lowest water content was reached at low water tables and low nutrient addition in our experiment, but even the lowest water content was far above 20%, so active drying of the biomass is necessary for most applications. Again, the trade-off between high productivity and short drying time requires prioritization of one over the other regarding desired growing conditions. For paper and new utilization options like biodegradable packaging, a high cellulose content close to 40% and low lignin content below 30% are required (Ververis et al., 2004). We found Typha biomass to be suitable for paper production: In our study, both species contained 35%-50% cellulose and 10%-13% lignin. T. angustifolia had significantly higher cellulose content at high water tables, whereas the water table had no influence on lignin content in either species.
Along the nutrient addition gradient, cellulose content was optimal at medium nutrient addition (approx. 20 kg N ha −1 a −1 ) while lignin content decreased with increasing nutrient addition. If packaging is to be used in the food sector, the absence of harmful substances such as herbicides and heavy metals is another requirement. Here, we did not test for herbicides and heavy metals as their occurrence in our mesocosm setting, far from agricultural landscapes, would not be transferable to field conditions. Such substances since the mesocosms were closed systems without contact with surface water or agricultural fields that would have exposed them to herbicides or heavy metals.
In addition to the factors investigated in this study, water table and nutrient supply, there are other factors that can strongly influence yield, tissue chemical composition, and water content of Typha biomass in field cultivation, among them are harvest date, stand age, water level fluctuations, interspecific competition and genotype (Geurts et al., 2020;Kasak et al., 2020;McNaughton, 1974;Tanaka et al., 2004). For high-value utilization, which was the focus of this study, biomass is preferably harvested in winter (Wichtmann, 2016). However, aboveground biomass harvested in winter reaches a lower yield and generally has lower nutrient concentrations, lower fiber, and higher protein content, than summer harvested biomass, as biomass allocation and nutrients are shifted to belowground organs towards winter Kasak et al., 2020;Pijlman et al., 2019). Furthermore, in the first years after planting, the productivity of Typha stands increased with increasing stand age and, in other paludicrops, ash content increased while the content of elements relevant for combustion decreased Giannini et al., 2016).

| Insights for paludiculture
In this study, we found that Typha productivity was surprisingly tolerant of low nutrient supply and its biomass quality even benefitted under such conditions. Low nutrient levels are to be expected at established paludiculture sites after several seasons with a yearly nutrient removal by harvest (Geurts et al., 2020;Hinzke et al., 2021).
As the levels with low nutrient addition in this study mimic such a nutrient-poor field site and the Typha plants reached comparable aboveground productivity to established stands, despite their young age and possibly lower nutrient storage capacity in the rhizome, we conclude that it is possible to maintain an unfertilised paludiculture in the long term and thus counteract eutrophication of the environment. In paludiculture, rewetting is primarily aimed at providing urgently needed climate benefits in the form of C sequestration, reducing greenhouse gas emissions, and providing other ecosystem services such as water storage and improvement of water quality by removal of nutrients and pollutants (Bhatia & Goyal, 2014;Biancalani et al., 2014;Dion & McCandless, 2013;Grosshans, 2014). This can only be achieved by keeping the water table at soil surface and above, which is a prerequisite for paludiculture. Due to smallscale soil unevenness, rewetting of formerly drained peatlands will result in a mosaic of different water table depths. In a large-scale field cultivation, however, it is not possible to make such small-scale differences in crop cultivation or utilization to ensure optimal cultivation everywhere. Therefore, species that provide a consistent quality over a wide range of water table depths are desirable. Our finding that biomass yield of T. angustifolia and tissue chemical composition of T. latifolia are largely unaffected by water table depth are therefore promising results. Low water tables were beneficial for T. latifolia productivity, while high water tables were beneficial for T. angustifolia biomass quality.
With regard to utilization, our results imply that when plant size (diameter) or biomass yield is important, e.g., for certain building materials, T. latifolia is generally preferable over T. angustifolia. When a low content of macronutrients (N, P, K), high content of C, Si, and high C/N-ratio are beneficial, both species should be cultivated at low nutrient supply and T. angustifolia is preferable at high water tables. When cellulose and lignin content are important, e.g., for paper and biodegradable packaging, both species should be cultivated at medium nutrient supply (~20 kg N ha −1 a −1 ) and T. angustifolia is again preferable at high water tables.

ACK N OWLED G EM ENTS
We acknowledge funding of the PRIMA project by the Agency for We are grateful to Laurenz Teuber for valuable input to the experimental design and implementation, the gardeners of the university for much appreciated help with the plants and practical work, and our student assistants for their help. We thank all members of the PRIMA project and the Experimental Plant Ecology lab for continuing support and discussions about the experiment and this manuscript. Open Access funding enabled and organized by Projekt DEAL.

CO N FLI C T O F I NTE R E S T
The authors have no conflict of interest to declare.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data used in this study for direct analysis and visualization are available in Figshare of Typha angustifolia (blue) and Typha latifolia (red) along the water table gradient. Negative water tables are below soil surface and positive water tables above soil surface. Dots are original data points, and lines show the smoothed local polynomial regression fitting (loess). 95% confidence intervals are displayed in gray, and 83% confidence intervals are displayed in color. Shaded triangles next to plots indicate preferred (= dark) biomass properties for high-value utilization.

F I G U R E A 2
Number of leaves per shoot (span = 2.0) measured on 18 th September 2019, at the end of the growing season, of Typha angustifolia (blue) and Typha latifolia (red) along the water table gradient. Negative water tables are below soil surface and positive water tables above soil surface. Dots are original data points, and lines show the smoothed local polynomial regression fitting (loess, span = 1.5). 95% confidence intervals are displayed in gray, and 83% confidence intervals are displayed in color. The shaded triangle next to the plot indicates preferred (= dark) biomass properties for high-value utilization.
F I G U R E A 3 C content [g kg −1 dry matter] (span = 3.0) in aboveground biomass of Typha angustifolia (blue) and Typha latifolia (red) along the water table gradient. Negative water tables are below soil surface and positive water tables above soil surface. Dots are original data points, and lines show the smoothed local polynomial regression fitting (loess, span = 1.5). 95% confidence intervals are displayed in gray, 83% confidence intervals are displayed in color. The shaded triangle next to the plot indicates preferred (= dark) biomass properties for high-value utilization.
F I G U R E A 4 (a) Rhizome (span = 1.4) and (b) root (span = 1.4) dry weight [g] of Typha angustifolia (blue) and Typha latifolia (red) along the nutrient addition gradient. Dots are original data points, and lines show the smoothed local polynomial regression fitting (loess). 95% confidence intervals are displayed in gray, and 83% confidence intervals are displayed in color. Shaded triangles next to plots indicate preferred (= dark) biomass properties for high-value utilization.

F I G U R E A 5
Number of leaves per shoot (span = 1.5) measured on 18 th September 2019, at the end of the growing season, of Typha angustifolia (blue) and Typha latifolia (red) along the nutrient addition gradient. Dots are original data points, and lines show the smoothed local polynomial regression fitting (loess). 95% confidence intervals are displayed in gray, and 83% confidence intervals are displayed in color. The shaded triangle next to the plot indicates preferred (= dark) biomass properties for high-value utilization.
F I G U R E A 6 C content [g kg −1 dry matter] (span = 1.7) in aboveground biomass of Typha angustifolia (blue) and Typha latifolia (red) along the nutrient addition gradient. Dots are original data points, and lines show the smoothed local polynomial regression fitting (loess). 95% confidence intervals are displayed in gray, and 83% confidence intervals are displayed in color. The shaded triangle next to the plot indicates preferred (= dark) biomass properties for high-value utilization. Missing values are due to the fact that sufficient biomass was lacking for the analyses because the plants were growing poorly or had died.